Interleukin-6 (IL-6) has a central role in regulating the immune response and inflammation. IL-6 has a range of biological activities, acting in an endocrine, autocrine, and paracrine fashion on a diverse number of target cells. The major sources of IL-6 are monocytes. In addition, fibroblasts and endothelial cells along with T cells and B cells, keratinocytes, mesangial cells, and several tumor cells produce IL-6. Its synthesis is induced by IL-1, IL-2, TNF-β and interferons. IL-6 is inhibited by IL-4, IL-10, and IL-13. IL-6 thus plays an important role in immune regulation and inflammation as well as autoimmune diseases.
IL-6 has the ability to stimulate the final stages of B lymphocyte maturation, whereby B lymphocytes differentiate into mature plasma cells and secrete immunoglobulins (Ig). In addition, IL-6 induces IgE production and enhances T cell growth and cytotoxic T cell differentiation through augmentation of IL-2 receptor expression and IL-2 production. IL-6 also induces the secretion of hepcidin and promotes the production of vascular endothelial growth factor (VEGF). IL-6 is a growth factor for both tumor cells, including multiple myeloma and renal carcinoma cells, and non-tumor cells, including mesangial cells.
IL-6 synergizes with other cytokines to support bone marrow stem cell maturation, is a neutrophil activator and stimulates the production of platelets from megakaryocytes. It is also a potent inducer of terminal macrophage and osteoclast differentiation. IL-6 also shares several activities with IL-1 and TNF, including the induction of pyrexia and the production of acute phase proteins such as serum amyloid A, C reactive protein (CRP), alpha 1 antitrypsin, fibrinogen, and haptoglobulin. IL-6 and IL-3 synergize to support the formation of multilineage blast-cell colonies.
IL-6 mediates several unique anti-inflammatory effects. Where both IL-1 and TNF-α induce synthesis of each other, as well as IL-6, IL-6 terminates this upregulatory inflammatory cascade and inhibits IL-1 and TNF-α synthesis. IL-6 is also important as a cell growth factor that can induce proliferation of epidermal keratinocytes and mesangial cells and has an active role in mesangial proliferative glomerulonephritis.
The receptor complex for IL-6 (“IL-6 Receptor Complex”) belongs to the type I cytokine receptor superfamily and consists of two chains, IL-6R (80 kDa glycoprotein) and gp130. IL-6R is the ligand specific binding component while gp130 is a shared receptor component responsible for transmitting intracellular signals of IL-6 related cytokines such as leukemia inhibitory factor (LIF), ciliary neurotropic factor, oncostatin M, and IL-11.
The binding of IL-6 to its receptor leads to dimerization of gp130, resulting in the activation of gp130 associated kinase JAK1 and subsequently the tyrosine phosphorylation of gp130. Phosphorylation of gp130 mediates recruitment of STAT3 proteins, leading to enhanced transcription of many genes, including those that encode acute phase proteins. Whereas gp130 is ubiquitously expressed, IL-6R is more restricted. For example, IL-6R is expressed on immunocompetent cells and hepatocytes. Both IL-6R and gp130 have soluble forms. The complex of IL-6 and soluble IL-6R can act on cells expressing only gp130. This is potentially an important proinflammatory mechanism for soluble receptors.